1. Field of the Invention
The present invention generally relates to a wireless communication system, and more particularly to an apparatus and method for efficient interference cancellation in a wireless cellular system.
2. Description of the Related Art
Technologies used to provide users with data services in current wireless communication environments are classified into 2.5th or 3rd generation cellular mobile communication technologies such as Code Division Multiple Access 2000 1× Evolution Data Optimized (CDMA2000 1×EVDO), General Packet Radio Services (GPRS) and Universal Mobile Telecommunication Service (UMTS) and wireless Local Area Network (LAN) technologies such as an Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless LAN and the like.
A distinctive feature of the 3rd generation cellular mobile communication technology for providing a voice service via a circuit network is that packet data services are provided to users for accessing the Internet in a broad-band wireless communication environment.
With the development of mobile communication technologies, various LAN wireless access technologies such as an IEEE 802.16 based wireless LAN and the like are appearing. These technologies do not ensure a mobility level equal to that of a cellular mobile communication system. However, the LAN wireless access technologies are being proposed as an alternative plan for providing a high-speed data service in a wireless environment by replacing a wired communication network such as a cable modem or Digital Subscriber Line (xDSL) with a wireless LAN in a home network environment or a hot spot area such as a public place or school.
When the above-described wireless LAN provides high-speed data service, there are limitations in providing users with public network services due to propagation interference as well as very limited mobility and narrow service coverage.
Thus, efforts for overcoming the limitations are being made at various angles. For example, a large amount of research is being conducted on a mobile Internet technology for making up for the weak points of the cellular mobile communication system and the wireless LAN. Moreover, active research is being conducted on a Wireless Broadband Internet (WiBro) system as a representative example of the mobile Internet technology currently being standardized and developed. The WiBro system can provide high-speed data service using various types of Mobile Stations (MSs) in indoor/outdoor stationary environments and mobile environments at pedestrian speed and medium/low speed (of about 60 Km/h).
On the other hand, active research is being conducted to provide users with various classes of Quality of Service (QoS) in the above-described wireless communication system. A high-speed, high-capacity communication system is required which can process and transmit various types of information such as video data, wireless data, and the like beyond a voice-centric service. Thus, in the wireless communication system, a channel coding scheme proper for increasing system transmission efficiency and improving system performance is essential.
However, inevitable error and information loss may occur due to many factors such as multipath interference, shadowing, propagation attenuation, time variant noise and interference fading in wireless channel environments of the wireless communication systems which are different from wired channel environments. An actual transmitted signal may be significantly distorted due to the information loss. This distortion may be a factor degrading the overall performance of a wireless communication system.
As described above, an interfering signal may be generated in the wireless communication system. The interfering signal significantly affects system performance. For example, a cell edge area can be considered in a system in which a frequency reuse factor is 1. In the cell edge area, there is a signal transmitted from a Base Station (BS), for example, a serving BS, for receiving information from an MS connected thereto and a signal transmitted from a neighbor BS to another user using the same frequency. The signal from the neighbor BS directly interferes with the MS, resulting in the degradation of reception performance of the MS.
Conventionally, a broadband wireless communication system, for example, a cellular Orthogonal Frequency Division Multiplexing (OFDM) based system, employs a frequency reuse factor of 1 in order to improve spectral efficiency and facilitate cell planning. However, in a cell edge overlap area, the MS receives the signal of the serving BS overlapping with the interfering signal of the neighbor BS at similar power levels. Thus, there is a problem in that the link performance of the MS may be significantly degraded due to mutual interference of neighbor BSs.
To overcome this problem, a low code rate and low-order modulation scheme such as Quadrature Phase Shift Keying (QPSK) is used when a signal is transmitted to an MS located in the cell edge area. For example, Forward Error Correction (FEC) and repetition coding can be simultaneously employed for transmission. However, in this case, there is a problem in that spectral efficiency is excessively lowered.
An alternative method is a technology in which the MS detects a target signal by eliminating an interfering signal received from the neighbor BS using an interference cancellation technique. In this case, the complexity of the MS increases since the signal of the neighbor BS is to be demodulated for the interference cancellation. However, this technology is advantageous in that link performance may be significantly improved without degrading spectral efficiency.
FIG. 1 is a schematic diagram illustrating a structure of a receiver using an interference cancellation technique in a conventional wireless communication system.
FIG. 1 illustrates an example of one interfering signal. As illustrated in FIG. 1, there can be provided multiple transmitters 101 and 103, an interference canceller 111, a mixer 113, a channel estimator 115, an equalizer 117, a symbol demapper 119 and a decoder 121.
Referring to FIG. 1, the receiver 110 receives a signal in which a target signal S1 of the transmitter 101, for example, a serving BS, overlaps with an interfering signal S2 of the transmitter 103, for example, a neighbor BS. The receiver 110 first performs channel estimation and equalization on the interfering signal S2 using the interference canceller 111 and then generates signal points on a constellation using a symbol demapper within the interference canceller 111. The symbol demapper indicates an input complex value on a complex plane and generates a predetermined number of signal points.
Then the interference canceller 111 performs multiplication by a channel estimate h2 using the generated signal points and regenerates the interfering signal. The mixer 113 subtracts the interfering signal S2 from the received signal, thereby eliminating the interfering signal S2 from the received signal.
Using a signal obtained by eliminating a component of the interfering signal S2 from the received signal, the receiver 110 performs channel estimation and equalization on the target signal S1 using the channel estimator 115 and the equalizer 117. The symbol demapper 119 and the decoder 121 detect and decode the target signal, thereby demodulating the target signal.
To demodulate an interfering signal in a wireless communication system, for example, an OFDM system, the receiver should know in advance a modulation level of the interfering signal, for example, the presence of the interfering signal and a Modulation and Coding Scheme (MCS) level. Moreover, the receiver should be able to perform channel estimation of a signal received from the neighbor BS using a pilot of the transmitter 103, for example, the neighbor BS.
For conventional channel estimation on the interfering signal received from the neighbor BS as described above, the MS can perform the channel estimation only when knowing in advance a pilot pattern of the neighbor BS. That is, an interference cancellation process is performed based on a design of a system-operating scenario when a modulation level of the interfering signal is assumed to be, for example, QPSK. To conventionally detect the presence of the interfering signal, resource allocation information of the neighbor BS should be known in advance. The resource allocation information can be detected after receiving in advance a signalling channel of the neighbor BS and decoding the received signaling channel.
For the above-described channel estimation, the MS should know in advance the pilot pattern and the presence of the interfering signal from the neighbor BS. However, in this case, there is a problem in that the complexity of the MS may increase and the use of the interference cancellation technique may not be facilitated when the signaling channel itself is received.